The Passive Film on Aluminum

As with other active-passive-type metals and alloys, the pitting corrosion of aluminum and its alloys results from the local penetration of a passive oxide film in the presence of environments containing specific anions, particularly chloride ions. The oxide film is y-Al203 with a partially crystalline to amorphous structure (Ref 13, 59). The film forms rapidly on exposure to air and, therefore, is always present on initial contact with an aqueous environment. Continued contact with water causes the film to become partially hydrated with an increase in thickness, and it may become partially colloidal in character. It is uncertain as to whether the initial air-formed film essentially remains and the hydrated part of the film is a consequence of precipitated hydroxide or that the initial film is also altered. Since the oxide film has a high ohmic resistance, the rate of reduction of dissolved oxygen or hydrogen ions on the passive film is very small (Ref 60). 

It is generally accepted that the passive film contains flaws that are the favored sites for pit initiation (Ref 13, 14, 60). The flaws occur predominately at sites of intermetallic phase particles in the substrate aluminum, particularly copper and iron-bearing intermetallics. Correlations have been made of flaw shape and distribution with these 

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Interface Potential and Pit Initiation. It is generally accepted that pit initiation occurs when the corrosion potential or potentiostatically imposed potential is above a critical value that depends on the alloy and environment. However, there is incomplete understanding as to how these factors (potential, material, and environment) relate to a mechanism, or more probably, several mechanisms, of pit initiation and, in particular, how preexisting flaws of the type previously described in the passive film on aluminum may become activated and/or when potential-driven transport processes may bring aggressive species in the environment to the flaw where they initiate local penetration. In the former case, the time for pit initiation tends to be very short compared with the initiation time on alloys such as stainless steels. Pit initiation is immediately associated with a localized anodic current passing from the metal to the environment driven by a potential difference between the metal/pit environment interface and sites supporting cathodic reactions. The latter may be either the external passive surface if it is a reasonable electron conductor or cathodic sites within the pit. 

Upon exposure to air, aluminum forms a chemically inert AI2O3 oxide film that is a rapidly forming self-healing film. Therefore, the passive film on aluminum, as well as the corrosion product layer, is a main barrier and leads to a resistant material in natural environments. 

The passive film on aluminum also consists of an inner layer of AI2O3 and an outer layer of AlOOH. 

A galvanic cell is formed when two metals differing in potential are joined together. For instance, if copper is joined to aluminum, aluminum would corrode because it has a more negative potential (—1.66 V) than copper (-f0.521 V). Copper being less active becomes the cathode and aluminum becomes the anode. But if iron is joined to aluminum, the iron corrodes (in seawater), due to the passive film on aluminum which causes it to behave like a nobler metal than iron (but not nobler than copper). The formation of such galvanic cells often leads to the corrosion of underground buried structures. A steel plate with copper rivets... 

Lithium bis(oxalato)borate (LiBOB) shows only moderate solubility up to about 1.0 M in some organic solvents (such as blends of PC and EC). Its conductivity is about 8-9 mS cm in appropriate solvents [97] (in DME even 14.9 mS cm at ambient temperature [98]). A major advantage is its thermal stability (up to 300 °C [99]) and the passivation film on aluminum, formed by the first cycle. This passivation film protects the aluminum current collector even at higher potentials than LiPFfi does, without breakdown up to 5.75 V [97, 100]. Furthermore, LiBOB has slightly better cycHng stability at ambient temperature, which is considerably increased at temperatures up to 70 °C [97]. Another advantage is that LiBOB forms... 

L. Tomcsanyi, K. Varga, I. Bartik, G. Horanyi, and E. Maleczki, Electrochemical Study of the Pitting Corrosion of Aluminum and Its Alloys II, Study of the Interaction of Chloride Ions with a Passive Film on Aluminum and Initiation of Corrosion, Electrochim. Acta, Vol 34, 1989, p 855-859... 

Fig. 8 Adsorption of (a, b) sulfate and (c, d) chloride on the passive film of aluminum in 0.1 M NaCl04 solution at different pH and open circuit potential, (a) Sulfate and (c) chloride surface concentration versus time at different pH...

Lopez et al. [160] applied the technique of Fermi level shift monitoring to characterize the add base properties of passive films on aluminum. The decreasing trend of relative basicity was found to be boehmite > thermal oxide > NaOH-degreased surface > silicate containing detergent-degreased surface > phosphoric add anodic film. The... 

Penetration of chloride ions this mechanism [first discussed by Hoar et al. (1965)] involves, following the adsorption of Cl" on the passive film surface, the entry of Cl" into the film and its transport through the passive film to the metal/oxide interface, where it causes breakdown of the passive film. The accumulation of Cl" at the interface or the formation of metal chloride may cause the film breakdown. Support of this mechanism is provided by the observation of chlorides in the inner oxide part of the passive film on nickel (Marcus and Herbelin, 1993), Fe-Cr (Yang et al., 1994), and aluminum (Natishan etal., 1997). 

More recently, Vertes and coworkers have investigated the spectral properties of the passive film on tin in borate buffer medium (pH 8.4) over a wide potential range. A typical voltammetric curve for Sn in this specific electrolyte is shown in Figure 23. At -0.9 V, the transmission Mossbauer spectrum of a " Sn-enriched tin film, electrodeposited on an aluminum substrate (curve A, Figure 24), was found to have clear contributions due to )8-Sn, 8 = 2.5, and Sn02 or Sn(OH)4, 8 = 0.03 (Table XI). The small absorption peak at about 4.2 mm s was attributed to the high-velocity component of a... 

Bonhoeffer, Vetter, and others (63) have made extensive studies on iron which indicate that the passive film is composed of one or more oxides of iron. Young (64). Vermilyea (65) and Johansen et at, (57) have shown that the Mott-Cabrera concepts are applicable for the thin films on Ta, Ti, Hf, and Hb. Petrocelli (58) has shown evidence that the dissolution of aluminum In sulfuric acid takes place through a thin film and that the process appears to follow the Motr-Cabret a theory. Stern (66) reports data indicating that the kinetic. for the anodic oxidation of stainless steel are similar to those for aluminum apd tantalum (67). Pryor (68) has recently reviewed the work on passive films on iron and suggests a single passive film of y contains non-uniform defect concentra-... 

Yet, for systems A and C, the measured fracture energies remain low compared with the critical fracture energy of the bulk aluminum 10 J Moreover, we do not observe islands of passivation material on the A1 fracture surface and, inversely, we do not observe A1 on debonded surfaces of the passivation films. This suggests that the loss of interfacial adhesion is close to a brittle fracture process despite the influence of plasticity of the A1 substrate and crack blunting at the interface. This sort of brittle mode of interfacial failure, including plastic flow in a ductile material (the substrate), has been observed or discussed for a sapphire/Au interface. ... 

With the more active metals, such as aluminum, titanium, and tantalum, oxide films form immediately on contact with air and behave as passive films in aqueous solutions. In the case of tantalum, the passive film is protective over the entire pH range in other cases, the films may... 

The precise mechanism resppnsible for the passivity conferred on metals by anodic inhibitors, such as chromate, is not known. While some early workers thought that a protective salt film (e.g., chromate) was formed, this view is not generally applicable, since passivity can occur in a system where the salt film would be freely soluble (e.g., iron in nitric acid). It is, however, generally accepted that passivity is associated with the formation of a protective film, and current views ascribe the action of anodic inhibitors either to adsorption at anodic sites or to continuous repair of the protective film. The former view has received attention in recent publications by Cartledge ), while the latter is favored by Evans (2). However, work on aluminum has suggested that true passivity is associated with the crystal structure of the film, which in turn determines its stability. This principle has recently been introduced by one of the authors (3) and is developed below into a general theory of passivity. 

Finally, several film-formation mechanism can be active in parallel for a given inhibitor/metal system. As an example, radiotracer experiments by Canes and coworkers on the accumulation of phosphate on aluminum are reproduced in Fig. 10 [42, 43]. After a first rapid increase in phosphate surface concentration, which was assigned to phosphate adsorption and (partial) incorporation into the passive film, a slower accumulation is observed, which can continue up to several days and which was explained by precipitation... 

Other metals that have favorable reversible Flade potentials and form passive film on their surfaces include titanium, silicon, aluminum, tantalum, and niobium. Naturally formed aluminum oxide protects the underlying aluminum metal at pH between 4 and 8. Titanium possesses very high oxidizing potentials and is used to manufacture anodes for cathodic protection systems for the chlorine-alkafi process (production of hydrogen, chlorine, and sodium hydroxide) and many other appfications. 

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